On 22 Jan 2020, Circle released the Dec 2019 report attesting to the US dollar reserves backing USDC by independent accounting firm, Grant Thornton LLP. They are the same accounting firm that is overwatching the Cryptopia.nz exchange hack and the accounting of the remaining funds.
As of December 31, 2019 at 11:59 PM Pacific Time (PT):
USDC has issued (and outstanding) 519,628,995 USDC tokens.
$520,537,729 US Dollars (USD) are held in custom accounts covering the outstanding tokens issued.
Full report is here:
https://www.centre.io/pdfs/attestation/grant-thornton_circle_usdc_reserves_20200116.pdf
If you want to download past reports, they are available here:
https://www.centre.io/usdc-transparency
In the comments below, explain why Circle is performing a monthly report?
How does this compare to Tether? Gemini Dollar? DAI?